Comprehensive characterization and phylogenetic analysis of the complete plastomes of two ant-orchids, Caularthron bicornutum and Myrmecophila thomsoniana.

Background: Myrmecophytes, characterized by specialized structures like hollow stems that facilitate mutualistic relationships with ants, serve as an important system for studying ant-plant interactions and the adaptation mechanisms. Caularthron and Myrmecophila are exemplary myrmecophytes within Orchidaceae. Previous studies suggested a genetic relationship between these two genera, placing them within Laeliinae (Epidendreae), yet the precise phylogenetic positioning remained uncertain. The absence of available plastome resources has hindered investigations into plastome evolution and phylogeny.

Results: In this study, we sequenced and assembled the complete plastomes of Caularthron bicornutum and Myrmecophila thomsoniana to elucidate their plastome characteristics and phylogenetic relationships. The determined plastome sizes were 150,557 bp for C. bicornutum and 156,905 bp for M. thomsoniana, with GC contents of 37.3% and 37.1%, respectively. Notably, M. thomsoniana exhibited a distinctive IR expansion and SSC contraction, with the SSC region measuring only 4532 bp and containing five genes (ccsA, ndhD, rpl32, psaC, and trnL-UAG), a unique feature observed for the first time in Epidendreae. Comparative analyses with species from the related genus Epidendrum revealed that C. bicornutum plastome exhibited conserved genome size, GC content, gene content, and gene order. A total of 32 and 33 long sequence repeats, 50 and 40 tandem repeats, and 99 and 109 SSRs were identified in the plastomes of C. bicornutum and M. thomsoniana, respectively. The RSCU analysis demonstrated a consistent pattern in both plastomes, with 29 out of 30 codons with RSCU values greater than 1 featuring A/U at the third codon position. Leucine was the most prevalent amino acid, while Cysteine was the least common. Four potential DNA barcoding regions with Pi values exceeding 0.07, namely ycf1, ccsA-psaC, petN-psbM, and accD-psaI, were identified for subsequent phylogenetic reconstructions within Laeliinae. Phylogenetic analysis underscored the close relationships among Caularthron, Epidendrum, and Myrmecophila.

Conclusions: This study represents the first comprehensive analysis of the plastome characteristics of Caularthron bicornutum and Myrmecophila thomsoniana. Through our characterization and phylogenetic analyses, we unveiled the unique IR expansion/SSC contraction and further elucidated their phylogenetic positions. Our research contributes significant data and insights into the dynamic evolution of ant-orchid plastomes and the phylogeny of the Laeliinae.